1. Technical Field
This invention relates generally to a carrier frequency tracking phase-locked loop, and more specifically, to a carrier frequency tracking phase-locked loop incorporating a direct digital synthesizer for providing a wide band carrier tracking and clock recovery system.
2. Discussion of the Related Art
Phase-locked loops are known in the art of carrier frequency tracking systems, such as systems for tracking satellites, for providing demodulation functions requiring regeneration of coherent carriers and clock signals. These phase-locked loops provide an effective means to track and lock onto a carrier signal having a frequency which varies widely with time. In a tracking and demodulation mode, the phase-locked loop regenerates a frequency changing carrier wave by locking onto the carrier wave and producing a stable and virtually noiseless replica of it.
Known phase-locked loops generally require a voltage controlled oscillator to reproduce the carrier signal. A typical voltage controlled oscillator (VCO) has a maximum effective bandwidth of approximately one octave because of the necessity to tune or align the VCO to the desirable range of frequencies. If a single VCO is used to cover more of a bandwidth than one octave, spectral purity is lost due to the presence of strong harmonics and loss of substantial noise performance. In addition, to accurately tune a single VCO beyond one octave requires the ability to compensate for excessive variations in tuning linearity exhibited by the VCO. Therefore, it has been the case in prior art phase-locked loop tracking systems that in order to provide the required tracking over a wide range of frequencies multiple VCO's are required.
FIG. 1 shows a known phase-locked loop incorporating multiple VCO's. In that figure, the signal on input line 12 represents a broad band input having a frequency which changes with time. Input line 12 is applied to a phase detector 14 along with a return line 16 which provides a feedback signal. An output line 18 from phase detector 14 is applied as an input to a voltage integrator low-pass loop filter 20. An output line 22 from low-pass filter 20 is applied as an input to a gain compensation circuit 24. An output of gain compensation circuit 24 on line 26 is applied to a first switch 28. Switch 28 has a plurality of outputs which are connected to a plurality of VCO's 30. An output of each VCO 30 is applied to a second switch 32 which has single output "f.sub.out " on line 34. The feedback signal on line 16 is taken from output line 34 and applied to phase detector 14 to form the phase-locked loop.
In operation, phase detector 14 compares the phase of the input signal on line 12 and the feedback signal on line 16. The output on line 18 of phase detector 14 is a DC magnitude signal representative of this phase difference. Low-pass loop filter 20 actively removes any remaining AC components of the DC magnitude signal according to their frequencies. Gain compensation circuit 24 takes the pure DC signal on line 22 from low-pass loop filter 20 and provides gain compensation to modify the loop gain to a constant value regardless of the different loop gains and tuning parameters of the different VCO's or input signals. The output on line 26 of the gain compensation circuit 28 is applied to first switch 28 which selects the appropriate VCO depending on the frequency range of interest. Each VCO 30 takes the DC signal and converts it to a clean representative of the input signal on line 12. The rebuilt input signal is then applied to a second switch 32 which produces a single output. The output of switch 32 has a return line 16 applied to phase detector 14 as described above.
If the frequency of the input signal on line 12 changes, the change in phase of the input signal on line 12 and the locked signal on line 16 will modify the DC signal on line 18 to represent this phase change. This in turn will alter the output from VCO 30, or select a different VCO 30 depending on the change in phase, such that the frequency signal on return line 16 is matched to the frequency of the input signal 12. By this, the input signal on line 12 can be locked onto a certain input frequency such that a clean representative of this signal can be used by the system, such as an integrate and dump circuit, clocking system, etc., in which the phase-locked loop is incorporated. Therefore, noise, fading, etc. can be greatly eliminated and the carrier frequency can be effectively tracked.
FIG. 2 shows another prior art phase-locked loop tracking system 40 in which an input signal on line 42 having a frequency which changes in time is applied to a first switch 44. Switch 44 applies this input frequency signal to one of a plurality of different phase-locked loops 46 depending on its frequency Each phase-locked loop 46 represents the entire circuit of FIG. 1 with a single VCO. The output of each phase-locked loop 46 is applied to a second switch 47 to form a single output. This system is very costly, in that it requires multiply redundant hardware. In addition, each loop 46 must be separately tuned, aligned, and monitored for proper acquisition, tracking, stability and noise performance.
The phase-locked loop 10 of FIG. 1 requires less hardware than that of the system 40 of FIG. 2, but still requires the redundancy of multiple VCO's. Each VCO requires its own set of tuning characteristics and the addition of complex gain compensation circuitry to maintain proper loop performance.
Another known prior art tracking system similar to that of FIG. 1 is a single phase-locked loop having a single VCO with multiple varactor diode tuning circuits. This third system also requires vast amounts of tuning characteristics having gain compensation circuitry which varies with time, temperature, and radiation.
What is needed then is a single phase-locked loop capable of being tuned or aligned over a very broad band and requiring a large reduction in hardware and tuning requirements, and the cost associated with these reductions. It is therefore an object of this invention to provide a tracking system having very broad brand acquisition, high spectral purity and minimal hardware.